Solid state image sensors are in common use of late as the imaging components of video cameras. Broadcasting cameras, in particular, include a solid state image sensor of the so-called frame interline transfer type which comprises an imaging region consisting of photosensitive elements and vertical shift registers, a storage region adapted to store signal charges, a horizontal shift register and a buffer amplifier so that the signal charges stored in photosensitive elements during an integrating time may be transferred at high velocity during the vertical blanking interval.
The conventional solid state image sensor and its driving method are now described in some detail. FIG. 1 is a schematic view showing a representative prior art solid state image sensor. As shown, its imaging region 1 comprises a multiplicity of photosensitive elements 5 arranged in a matrix form and a plurality of vertical shift registers 6 adapted to transfer signal charges read out of the corresponding photosensitive elements 5 in a vertical direction. The storage region 2 comprises a plurality of vertical shift registers 7 which store the signal charges transferred from the imaging region 1 and line-transfer these signal charges to a horizontal shift register 3. The buffer amplifier 4 is a means for outputting the signal charges transferred to said horizontal shift register 3.
In the above prior art solid state image sensor, each vertical shift register 6 has two independent transfer electrodes per picture element and transfers a block of signal charges by means of four transfer electrodes as an independent unit. Thus, the number of stages of said vertical shift register 6 is one-half of the number of photosensitive elements 5 in one vertical column and the number of stages of the vertical shift register 7 comprising the storage region 2 is equal to the number of stages of said vertical shift register 6.
The method for driving the prior art solid state image sensor illustrated in FIG. 1 is now explained. FIG. 2(a), FIG. 2(b), FIG. 2(c) and FIG. 2(d) show the timing charts of the driving method for the prior art solid state image sensor. Thus, FIG. 2(a) represents the composite synchronizing pulse, FIG. 2(b) represents the vertical transfer pulse applied to the vertical shift register 6, FIG. 2(c) represents the vertical transfer pulse applied to the storage region 2, and FIG. 2(d) represents the horizontal transfer pulse applied to the horizontal shift register 3. Referring, first, to FIG. 2(a), the driving method comprises reading out the signal charges from the photosensitive elements 5 in sets of two each during the vertical blanking interval 21a of field A and the vertical blanking interval 21b of field B and transferring the signal charges to the storage region 2 through the vertical shift register 6, and outputting them as a picture signal from the buffer amplifier 4. This sequence is explained in further detail taking field A as an example. First, as shown in FIG. 2(b), the unnecessary charges accumulated in the vertical shift register 6 are swept by a sweeping pulse 22 in the vertical blanking interval 21a. Then, the signal charges accumulated in every other photosensitive elements 5 in the vertical direction are read out to the vertical shift register 6 by a read pulse 23. Then, the signal charges in the vertically adjacent photosensitive elements 5 are mixed by a mixing pulse 24 and transferred to the storage region 2 by a transfer pulse 25. Thereafter, as shown in FIG. 2(c), the signal charges are horizontally line-transferred, during each horizontal transfer interval, from the vertical shift register 7 of the storage region 2 to the horizontal shift register 3 by a line transfer pulse 26. Then, the unnecessary charges remaining in the vertical shift register 7 are swept by a sweeping pulse 27 and signal charges from the imaging region 1 are freshly transferred by a transfer pulse 28. Thereafter, as shown in FIG. 2(d), the signal charges are transferred to the horizontal shift register 3 by a horizontal transfer pulse 29 and outputted as a picture signal from the buffer amplifier 4. Regarding field B, a picture signal is outputted from the buffer amplifier 4 in the same manner as above except that signal charges from a different combination of photosensitive elements 5 from that used for field A are dealt with.
Now, the method for driving the storage region of the prior art solid state image sensor is explained.
FIG. 3(a) is a waveform diagram showing the driving signal applied to the storage region of the prior art solid state image sensor and FIG. 3(b) is a potential profile of the storage region of the same solid state image sensor. These diagrams pertain to the 4-phase driving mode and the drive pulses represented by o1, o2, o3 and o4 in FIG. 3(a) are applied to four transfer electrodes per group of the storage region 2. The potential profile across a vertical section of the vertical shift register 7 of the storage region 2 can be represented as FIG. 3(b). Thus, signal charges 31 are transferred from A to B to C to D. The maximum signal quantity that can be dealt with in the vertical shift register 7 of the storage region 2 is defined by the potential well formed by drive pulses o2 and o3 as shown at A in FIG. 3(b).
However, in the above solid state image sensor wherein signal charges from all the photosensitive elements are transferred in a lump sum to the storage region 2, the maximum signal quantity that can be handled is limited to the quantity of signals which the vertical shift register 6 can manage to transfer in one operation so that the dynamic range is inevitably low.
Furthermore, in the above solid state image sensor, the signal charges of all the photosensitive elements cannot be transferred independently in the discrete form to the storage region 2 within the vertical blanking interval, such that no progressive-scan can be achieved.